The invention relates to nuclear magnetic resonance (NMR) spectroscopy, and in particular to systems and methods for positioning NMR samples in NMR spectrometers.
Nuclear magnetic resonance (NMR) spectrometers typically include a superconducting magnet for generating a static magnetic field B0, and an NMR probe including one or more special-purpose radio-frequency (RF) coils for generating a time-varying magnetic field B1 perpendicular to the field B0, and for detecting the response of a sample to the applied magnetic fields. Each RF coil and associated circuitry can resonate at the Larmor frequency of a nucleus of interest present in the sample. The direction of the static magnetic field B0 is commonly denoted as the z-axis or longitudinal direction, while the plane perpendicular to the z-axis is commonly termed the x-y or transverse direction. The RF coils are typically provided as part of an NMR probe, and are used to analyze samples situated in sample tubes or flow cells.
In a commonly-used approach, NMR sample tubes are spun at high speeds (e.g. at a frequency of several kHz) during measurements in order to achieve narrower spectral linewidths. Spinning the samples causes the NMR measurements to reflect an azimuthal averaging of gradients and inhomegeneities in the magnetic fields applied to the samples. For solid polycrystalline samples, spinning also allows azimuthally averaging over the various crystalline orientations present in the sample. For solid samples, optimal spectra can be achieved when the angle between the spinning axis (the longitudinal sample axis) and the direction of the static magnetic field is the “magic angle,” or about 54°. The magic angle is the solution to the equation 3 cos2θ−1=0. Liquid or gaseous samples are typically spun about an axis coinciding with the direction of the static magnetic field.
In U.S. Pat. No. 3,991,627, Laird et al. describe a device for longitudinally aligning sample containers such as test tubes regardless of the containers' size. The device comprises a foot member provided with a tapered cavity for receiving the bottom end of the container, a holder for holding the other end of the container in a temporary position, and a head member having a tapered cavity for receiving the other or upper end of the container. The foot and head members are movable relative to one another so that the cavity of the head member can releasably engage the upper end of the container and align the container with a predetermined longitudinal axis.